Binding of Scandium Ions to Metalloporphyrin-Flavin Complexes for Long-Lived Charge Separation

A porphyrin-flavin-linked dyad and its zinc and palladium complexes (MPor-Fl: 2-M, M=2 H, Zn, and Pd) were newly synthesized and the X-ray crystal structure of 2-Pd was determined. The photodynamics of 2-M were examined by femto- and nanosecond laser flash photolysis measurements. Photoinduced electron transfer (ET) in 2-H₂ occurred from the singlet excited state of the porphyrin moiety (H₂Por) to the flavin (Fl) moiety to produce the singlet charge-separated (CS) state ¹(H₂Por^.+-Fl^.-), which decayed through back ET (BET) to form ³[H₂Por]∗-Fl with rate constants of 1.2×10¹⁰ and 1.2×10⁹ s^-1, respectively. Similarly, photoinduced ET in 2-Pd afforded the singlet CS state, which decayed through BET to form ³[PdPor]∗-Fl with rate constants of 2.1×10¹¹ and 6.0×10¹⁰ s^-1, respectively. The rate constant of photoinduced ET and BET of 2-M were related to the ET and BET driving forces by using the Marcus theory of ET. One and two Sc³⁺ ions bind to the flavin moiety to form the Fl-Sc³⁺ and Fl-(Sc³⁺)₂ complexes with binding constants of K ₁=2.2×10⁵ M ^-1 and K ₂=1.8×10³ M ^-1, respectively. Other metal ions, such as Y³⁺, Zn²⁺, and Mg²⁺, form only 1:1 complexes with flavin. In contrast to 2-M and the 1:1 complexes with metal ions, which afforded the short-lived singlet CS state, photoinduced ET in 2-Pd Sc³⁺ complexes afforded the triplet CS state (³[PdPor^.+-Fl^.--(Sc³⁺)₂]), which exhibited a remarkably long lifetime of τ=110 ms (k _BET=9.1 s^-1). More than transient: A porphyrin-flavin-linked dyad and its zinc and palladium complexes were synthesized and the X-ray crystal structure of the palladium complex was determined. The Pd-Sc³⁺ complex afforded a remarkably long lifetime of 110 ms of the charge-separated state (see figure).